The present invention relates to an optical transmission system utilizing the optical frequency division multiplexing technology for sending a large volume of information by means of a single optical fiber.
An optical frequency division multiplexing transmission system (the frequency division multiplexing is hereinafter referred to as "FDM") normally comprises an optical FDM transmitter equipment including a plurality of transmission optical sources (in the number of N, where N is a natural number of 1 or more), an optical fiber for transmitting an optical FDM signal and an optical FDM receiver including a plurality (a number N) of heterodyne optical receivers. The frequency interval of a transmission optical source is normally stabilized at a predetermined value of Dop. Each heterodyne optical receiver (the n-th one, for example, where n is a natural number satisfying the relation 1.ltoreq.n.ltoreq.N) receives only an optical signal outputted from a corresponding (n-th) optical source on transmitter side. A conventional optical FDM transmission system is described, for example in IEEE Photonics Technology Letters, Vol. 2, No. 12, pp. 914-916. In the conventional optical FDM transmission system, the frequency of a local optical source (hereinafter referred to as "the local frequency") is controlled in such a manner as to follow the variations in the central frequency of a received optical signal (hereinafter referred to as "the optical signal frequency"), thereby stabilizing the difference frequency (hereinafter referred as "the intermediate frequency") at a predetermined value f.sub.IF.